It is known in the shoe manufacturing industry that thermoplastic cements are utilized to join an outsole to the lasted upper. In the use of such cements, the solvent must be allowed to evaporate so the parts can be handled easily and thus avoid the chance of sticking together before the desired alignment is made. When the shoe parts are ready for assembly the cement is exposed to heat which causes it to soften and become tacky, whereupon the parts are aligned properly and clamped in a press while the cement cools below its softening point.
The most commonly used source of heat for activating the thermoplastic cement is developed from resistance type heating elements which heats the shoe parts primarily by convection. Cal rods are the most frequently used due to the favorable cost. Another common heating element is the glass panel or pyrex panel heating element in which low thermal expansion glass is made to have relatively low electrical resistance. When an EMF is impressed across the ends of the glass it becomes a large resistance heating surface so the shoe parts can be placed near by to activate the cement by both convection and conduction. The nature of the glass panel causes uneven heating in its surface area and that leads to uneven activation of the cement on the shoe parts.
In recent years, quartz lights have been used as the heating elements in outsole activators. These lights operate in the infrared energy range and develop temperature of the order of 3000.degree. F. in about one second of operation. Quartz lights have low mass and can cool quickly when compared to the cool down time of other heating means. However, quartz lights have a high watt density and generally heat objects mostly by radiant energy.
Heating elements of the quartz light type have advantages in the application to activation of the thermoplastic cement used in the manufacture of shoes. These advantages are: operator comfort from lack of heat blast when removing shoe parts from the heating area; flash heating from the rapid rise in temperature which does not unduly heat the shoe parts on which the cement is placed, ability to use shoe materials having a low melting point and avoidance of operator judgment; reduction in the time shoe parts need to remain in a press which improves productivity and ultimate cost; and low thermal inertia which allows heat delivery quickly at a high temperature and rapid cool down at zero energy consumption.
Commercial equipment for heat activating cement in the manufacture of shoes shows a wide diversity in designs. A British Profile Activator has a single table top with the quartz lights above the outsole, while the lasted upper is heated by a Cal Rod over the upper. A Compo activator has quartz lights in a chamber and a shuttle for moving the outsole into the chamber, while the uppers are Cal Rod heated on top of the chamber. American Shoe Machinery Activator is provided with two banks of quartz lights located beneath the outsole and the upper. A light shield protects the eyes of the operator, and a control activates both banks during the heating cycle.